Organic Petrographic And Organic Geochemical Studies Of The Sour Gas Accumulations In The Lower Triassic Feixianguan Formation, NE Sichuan Basin

Based on field observation, laboratory experiments and geological analogy analysis, this study used organic petrography and organic geochemistry to constrain thermochemical sulfate reduction (TSR) and natural gas origin for the sour gas accumulations in the Lower Triassic Feixianguan Formation, northeastern Sichuan Basin. The presence of highly heterogeneous reservoir bitumens in terms of both bitumen reflectance and S/C and O/C atomic ratios indicates that the reservoir bitumens have only been partially affected by TSR reactions, but the chemical and stable carbon isotopic signatures of gaseous alkanes remain largely genetically indicative. The identification of multiple types of gases occurring in the Feixianguan Formation reservoirs implies several discrete gas migration and accumulation conduits. The preferential leakage of methane from the early accumulated oils, in combination of a later addition of soluble gases from the water leg during the late stage uplift, could help explain why the ethane in the eastern and northeaster Sichuan Basin is exceedingly enriched in ¹²C. Several lines of evidence suggest the Upper Permian Longtan Formation as the main source rocks for the paleo-oil accumulation in the Feixianguan Formation, and the sour gas accumulations resulted from thermal oil cracking and TSR reactions. Minor contributions also come from inter-reefal lagoonal source rocks within the Feixianguan Formation and deeper strata. In addition to formation waters within the gas reservoirs and/or cap rocks, hydrothermal fluids derived from deep seated basement faults may also provide both heat and sulfate for TSR reactions in some of the studied reservoirs. It appears that sulfate supply is the main rate limiting step for the TSR reactions.